Input/Output (I/O) pins for electronic applications are typically attached to a semiconductor substrate, such as a ceramic substrate, using solder or braze, such as, for example, tin-antimony (SnSb) solder. In most cases tin-antimony solder can be used only with specially designed pins which have selective metal plating on the surface or base of the pins. The selective plating is an important factor as it limits the solder or braze to wet only the base and preferably the tapered portion of the pin (if the pin has a tapered portion) and not the shank of the pin. On the other hand, for some applications, a solder or braze wettable material, such as gold (Au), is required on a critical area of the shank for connection to the board, and on, the pin head, i.e., on the tapered side and the base of the pin, to obtain an ideal solder fillet shape.
One problem with the currently used I/O pins that are secured to the semiconductor substrate is that they have a poor solder fillet shape, as more clearly seen in FIG. 1, where an I/O pin 15, having a stem or shank 19, and a pin head 17, is secured to a bonding pad 12, on a semiconductor substrate 10, using a braze or solder material 14. In this case there is no solder or braze material on the tapered side of the pin head 17, and therefore the pin 15, cannot take the stresses as required for such pins and as more clearly discussed in U.S. Pat. No. 4,970,570, assigned to the assignee of the present invention and incorporated herein by reference.
Another problem that has been noticed with some of the currently used I/O pins, is that they are either made of a material that is wettable by solder or braze. Or that both the shank and the tapered side of the I/O pin head have a coating of gold. This could be as a result of either a coating of residual gold or the presences of a continuous gold layer. Solder run-up may thus occur, as more clearly shown in FIG. 2, where the solder or braze material 14, can be seen not only on the tapered side of the pin head 17, but also along portions of the shank 19. This creates more stresses and failure conditions for the pin 15.
One method of selectively electroplating gold on metallic surfaces containing copper using a chromate film as a plating mask is disclosed in U.S. Pat. No. 4,077,852. Prior to putting down the chromate mask or film certain areas may be masked by use of organic films such as stop-off lacquer or photoresist material.
Another process of selective plating is disclosed in U.S. Pat. No. 4,199,415, where a metal surface which will be selectively plated with a precious metal, includes the steps of producing a removable mask on the metal surface to be plated, which mask covers the area to be plated, and electroplating the metal surface in an electrolytic bath with an organic resin paint, such that the paint covers the portion of the metal surface not covered by said removable mask, removing the removable mask so as to leave the area to be plated with the precious metal exposed, and electro-plating the exposed area of the metal surface with the precious metal.
U.S. Pat. No. 4,280,882, discloses another method for electroplating selected areas of article and articles plated thereby. This Patent also teaches that the movement of an electrolyte up the surface can be greatly curtailed by establishing negative capillarity along the baffle faces. This negative capillarity can be accomplished by coating the faces of the baffle elements with a material exhibiting limiting wettability with respect to the electrolyte bath, such as polytetrafluoroethylene.
However, the process of this invention allows the depositing of gold selectively on the pin shank for connection to a board, and on the head or base of the pin for obtaining an ideal solder fillet, while having these two metal coated areas separated by a non-wettable (no gold) surface.
Furthermore, this invention is amenable to masking an area of a three dimensional member, whereas other techniques are only practical with a two dimensional plane or surface.